Newsgroups: sci.space.shuttle
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: More launch windows
Message-ID: <1989May10.052514.16299@utzoo.uucp>
Organization: U of Toronto Zoology
References: <3205@kalliope.rice.edu> <388@cybaswan.UUCP>
Date: Wed, 10 May 89 05:25:14 GMT

The latest issue of World Spaceflight News (US/Canada $35/12, elsewhere
$50/12, airmail everywhere; Box 98, Sewell NJ 08080-0098 USA; a superb
source of technical detail on shuttle missions, with a counterpart
["Planetary Encounter"] on planetary missions) includes the launch-window
chart for the Magellan mission.  The chart has time of day ascending up
the vertical and date ascending rightward on the horizontal.  The window
is primarily a broad stripe running down from upper left to lower right.
The stripe cuts off at April 24 on the left and May 23 on the right, which
looks like something like energy requirements for the Venus trajectory.
[WSN unfortunately didn't print a detailed *explanation* of the chart!]
There is also a nearly-horizontal line running across, sloping slightly
upward to the right, above which the lighting at Trans-Atlantic-Abort
sites is unacceptable.  So the window opens when the stripe crosses the
line on about April 28, and reaches full width about May 10.  The
boundaries of the stripe are fuzzy, with several contours marked "yaw
steering", which I assume means that launches near the boundaries of the
window require extra shuttle maneuvering to get into the desired parking
orbit.  (The amounts, in pounds, given for the contours are too big to
be Magellan's own fuel.)  Finally, although the stripe is almost straight,
from about May 8 to May 12 both boundaries bend downward and then
recover; this is marked "perturbation due to Moon".

Magellan itself also preferred a slight launch delay, because maximum
economy on the injection into the desired Venus orbit required leaving
Earth orbit after about May 3.  (It is possible that this is the "yaw
steering", if somebody has misplaced some decimal points and if one edge
of the stripe is interpreted as negative and the other as positive.)
This is mentioned in the text.
-- 
Mars in 1980s:  USSR, 2 tries, |     Henry Spencer at U of Toronto Zoology
2 failures; USA, 0 tries.      | uunet!attcan!utzoo!henry henry@zoo.toronto.edu